[X86] Limit the number of target specific nodes emitted in LowerShiftParts

The important part is the creation of the SHLD/SHRD nodes. The compare and the conditional move can use target independent nodes that can be legalized on their own. This gives some opportunities to trigger the optimizations present in the lowering for those things. And its just better to limit the number of places we emit target specific nodes.

The changed test cases still aren't optimal.

Differential Revision: https://reviews.llvm.org/D48619

llvm-svn: 335998